The HPPDs are enzymes which catalyse the reaction in which para-hydroxyphenylpyruvate (abbreviated herein as HPP), a tyrosine degradation product, is transformed into homogentisate (abbreviated herein as HG), the precursor in plants of tocopherol and plastoquinone (Crouch N. P. et al. (1997) Tetrahedron, 53, 20, 6993-7010, Fritze et al., (2004), Plant Physiology 134:1388-1400). Tocopherol acts as a membrane-associated antioxidant. Plastoquinone, firstly acts as an electron carrier between PSII and the cytochrome b6/f complex and secondly, is a redox cofactor for phytoene desaturase, which is involved in the biosynthesis of carotenoids.
Up to now, more than 700 nucleic acid sequences from various organisms present in NCBI database were annotated as coding for a putative protein having an HPPD domain including the sequences disclosed under the Q0SC92 and Q0SF39 accession numbers given in the UniProtKB/TrEMBL database as well as under the YP—703002 and YP—702005 accession numbers respectively given in the NCBI protein database. But for most of those, including the sequences corresponding to the accession numbers Q0SC92/YP—703002 and Q0SF39/YP—702005, it has not been proven that the proteins would have an HPPD enzymatic activity either in an in vitro assay or an in in planta approach, nor that such HPPD protein can confer herbicide tolerance to HPPD inhibitor herbicides when expressed in a plant. Several HPPD proteins and their primary sequences have been described in the state of the art, in particular the HPPD proteins of bacteria such as Pseudomonas (Rüetschi et al., Eur. J. Biochem., 205, 459-466, 1992, WO 96/38567), of plants such as Arabidopsis (WO 96/38567, Genebank AF047834), carrot (WO 96/38567, Genebank 87257), Avena sativa (WO 02/046387), wheat (WO 02/046387), Brachiaria platyphylla (WO 02/046387), Cenchrus echinatus (WO 02/046387), Lolium rigidum (WO 02/046387), Festuca arundinacea (WO 02/046387), Setaria faberi (WO 02/046387), Eleusine indica (WO 02/046387), Sorghum (WO 02/046387), Coccicoides (Genebank COITRP), of Coptis japonica (WO 06/132270), Chlamydomonas reinhardtii (ES 2275365), or of mammals such as mouse or pig. The corresponding sequences disclosed in the indicated references are hereby incorporated by reference.
Most plants synthesize tyrosine via arrogenate (Abou-Zeid et al. (1995), Applied Env Microb 41: 1298-1302; Bonner et al., (1995), Plant Cells Physiol. 36, 1013-1022; Byng et al., (1981), Phytochemistry 6: 1289-1292; Connely and Conn (1986), Z. Naturforsch 41c: 69-78; Gaines et al., (1982), Plants 156: 233-240). In these plants, the HPP is derived only from the degradation of tyrosine. On the other hand, in organisms such as the yeast Saccharomyces cerevisiae or the bacterium Escherichia coli, HPP is a tyrosine precursor, and it is synthesized by the action of an enzyme, prephenate dehydrogenase (hereinafter referred to as PDH), which converts prephenate to HPP (Lingens et al., (1967) European J. Biochem 1: 363-374; Sampathkumar and Morrisson (1982), Bioch Biophys Acta 701: 204-211). In these organisms, the production of HPP is therefore directly connected to the aromatic amino acid biosynthetic pathway (shikimate pathway), and not to the tyrosine degradation pathway.
Inhibition of HPPD leads to uncoupling of photosynthesis, deficiency in accessory light-harvesting pigments and, most importantly, to destruction of chlorophyll by UV-radiation and reactive oxygen species (bleaching) due to the lack of photo protection normally provided by carotenoids (Norris et al. (1995), Plant Cell 7: 2139-2149). Bleaching of photosynthetically active tissues leads to growth inhibition and plant death.
Some molecules which inhibit HPPD, and which bind specifically to the enzyme in order to inhibit transformation of the HPP into homogentisate, have proven to be very effective selective herbicides.
At present, most commercially available HPPD inhibitor herbicides belong to one of these four chemical families:    1) the triketones, e.g. sulcotrione [i.e. 2-[2-chloro-4-(methylsulfonyl)benzoyl]-1,3-cyclohexanedione], mesotrione [i.e.2-[4-(methylsulfonyl)-2-nitrobenzoyl]-1,3-cyclohexanedione]; tembotrione [i.e.2-[2-chloro-4-(methylsulfonyl)-3-[(2,2,2,-tri-fluoroethoxy)methyl]benzoyl]-1,3-cyclo-hexanedione]; tefuryltrione [i.e. 2-[2-chloro-4-(methylsulfonyl)-3-[[(tetrahydro-2-furanyl)methoxy]methyl]benzoyl]-1,3-cyclohexanedione]]; bicyclopyrone [i.e. 4-hydroxy-3-[[2-[(2-methoxyethoxy)methyl]-6-(trifluoromethyl)-3-pyridinyl]carbonyl]bicyclo[3.2.1]oct-3-en-2-one]; Benzobicyclon [i.e. 3-(2-chloro-4-mesylbenzoyl)-2-phenylthiobicyclo[3.2.1]oct-2-en-4-one]    2) the diketonitriles, e.g. 2-cyano-3-cyclopropyl-1-(2-methylsulphonyl-4-trifluoromethylphenyl)-propane-1,3-dione and 2-cyano-1-[4-(methylsulphonyl)-2-trifluoromethylphenyl]-3-(1-methylcyclopropyl)propane-1,3-dione;    3) the isoxazoles, e.g. isoxaflutole [i.e.(5-cyclopropyl-4-isoxazolyl)[2-(methylsulfonyl)-4-(trifluoromethyl)phenyl]methanone]. In plants, the isoxaflutole is rapidly metabolized in DKN, a diketonitrile compound which exhibits the HPPD inhibitor property; and    4) the pyrazolinates, e.g. topramezone [i.e.[3-(4,5-dihydro-3-isoxazolyl)-2-methyl-4-(methylsulfonyl) phenyl](5-hydroxy-1-methyl-1H-pyrazol-4-yl)methanone], and pyrasulfotole [(5-hydroxy-1,3-dimethylpyrazol-4-yl(2-mesyl-4-trifluaromethylphenyl)methanone]; pyrazofen[2-[4-(2,4-dichlorobenzoyl)-1,3-dimethylpyrazol-5-yloxy]acetophenone].
These HPPD-inhibiting herbicides can be used against grass and/or broad leaf weeds in crop plants that display metabolic tolerance, such as maize (Zea mays) in which they are rapidly degraded (Schulz et al., (1993). FEBS letters, 318, 162-166; Mitchell et al., (2001) Pest Management Science, Vol 57, 120-128; Garcia et al., (2000) Biochem., 39, 7501-7507; Pallett et al., (2001) Pest Management Science, Vol 57, 133-142). In order to extend the scope of these HPPD-inhibiting herbicides, several efforts have been developed in order to confer to plants, particularly plants without or with an underperforming metabolic tolerance, a tolerance level acceptable under agronomic field conditions.
Besides the attempt of by-passing HPPD-mediated production of homogentisate (U.S. Pat. No. 6,812,010), overexpressing the sensitive enzyme so as to produce quantities of the target enzyme in the plant which are sufficient in relation to the herbicide has been performed (WO96/38567). Overexpression of HPPD resulted in better pre-emergence tolerance to the diketonitrile derivative (DKN) of isoxaflutole (IFT), but tolerance was not sufficient for tolerance to post-emergence treatment (Matringe et al., (2005), Pest Management Science 61: 269-276).
A third strategy was to mutate the HPPD in order to obtain a target enzyme which, while retaining its properties of catalysing the transformation of HPP into homogentisate, is less sensitive to HPPD inhibitors than is the native HPPD before mutation.
This strategy has been successfully applied for the production of plants tolerant to 2-cyano-3-cyclopropyl-1-(2-methylsulphonyl-4-trifluoromethylphenyl)-propane-1,3-dione and to 2-cyano-1-[4-(methylsulphonyl)-2-trifluoromethylphenyl]-3-(1-methylcyclopropyl)propane-1,3-dione (EP496630), two HPPD-inhibiting herbicides belonging to the diketonitriles family (WO 99/24585). Pro215Leu, Gly336Glu, Gly33611e, and more particularly Gly336Trp (positions of the mutated amino acid are indicated with reference to the Pseudomonas HPPD) were identified as mutations which are responsible for an increased tolerance to pre-emergence treatment with these diketonitrile herbicides without causing an alteration of the activity of the enzyme.
More recently, introduction of a Pseudomonas HPPD gene into the plastid genome of tobacco and soybean has shown to be more effective than nuclear transformation, conferring even tolerance to post-emergence application of isoxaflutole (Dufourmantel et al., 2007, Plant Biotechnol J.5(1):118-33).
In WO 04/024928, the inventors have sought to increase the prenylquinone biosynthesis (e.g., synthesis of plastoquinones, tocopherols) in the cells of plants by increasing the flux of the HPP precursor into the cells of these plants. This has been done by connecting the synthesis of said precursor to the “shikimate” pathway by overexpression of a PDH enzyme. They have also noted that the transformation of plants with a gene encoding a PDH enzyme makes it possible to increase the tolerance of said plants to HPPD inhibitors.
In the patent application WO 2009/144079, a nucleic acid sequence encoding a mutated hydroxyphenylpyruvate dioxygenase (HPPD) at position 336 of the Pseudomonas fluorescens HPPD protein and its use for obtaining plants which are tolerant to HPPD inhibitor herbicides is disclosed.
In WO 2002/046387, several domains of HPPD proteins originating from plants have been identified that may be relevant to confer tolerance to various HPPD inhibitor herbicides but no in planta nor biochemical data have been shown to confirm the impact of the as described domain functions.
In WO 2008/150473, the combination of two distinct tolerance mechanisms—a modified Avena sativa gene coding for a mutant HPPD enzyme and a CYP450 Maize monooxygenase (nsf1 gene)—was exemplified in order to obtain an improved tolerance to HPPD inhibitor herbicides, but no data have been disclosed demonstrating the synergistic effects based on the combination of both proteins.
Despite these successes obtained for the development of plants showing tolerance to several HPPD inhibitors herbicides described above, it is still necessary to develop and/or improve the tolerance of plants to newer or to several different HPPD inhibitors, particularly HPPD inhibitors belonging to the classes of the triketones (e.g. sulcotrione, mesotrione, tembotrione, benzobicyclon and bicyclopyrone) and the pyrazolinates (e.g., topramezone and pyrasulfotole).